Numerous applications require optical sources capable of generating optical output at stable and controllable wavelengths. For example, semiconductor laser diodes capable of delivering optical signals at stable and controllable wavelengths have become essential in wavelength-division multiplexing (WDM) and dense wavelength-division multiplexing (DWDM) telecommunications. Like most solid state sources, semiconductor laser diodes experience shifts in output wavelength due to temperature variation, changes in threshold current, degradation and/or aging. Such shifts in wavelength may result in disturbances affecting the operation of entire telecommunications networks.
In a practical optical telecommunications network based on dense wavelength-division multiplexing (DWDM), the light from several semiconductor lasers is combined onto a single optical fiber. To prevent cross-talk between these signals, it is crucial that each of the lasers in the system be tuned to a different wavelength. More specifically, the frequencies of the individual lasers are selected from a specific grid of frequencies (called the I.T.U. frequency grid).
There are several methods and devices, often referred to as wavelength-locking and wavelength lockers, that produce outputs proportional to the difference between an input light frequency and the ITU grid frequencies. Wavelength lockers utilize one or more optical filters such as transmission filters, reflection filters, interference filters, Fabry-Perot etalons, etc. and associated detectors to provide wavelength readout. These approaches tend to be simple, but they have a limited wavelength resolution. In general, the wavelength readouts obtained are translated into an error signal, and this error signal is used as a feedback signal to tune the laser temperature, current, or other operating parameter to keep the laser frequency near the desired grid frequency.
Some examples of prior art wavelength lockers that use one or more optical filters to provide wavelength readout include U.S. Pat. Nos. 4,815,081; 6,122,301; 6,400,737; 6,289,028 and 4,172,663. More specifically, U.S. Pat. No. 4,815,081 to Mahlein et al. teaches the use of a first optical detector device and a wavelength selective optical filter. The part of the power that passes through the filter is delivered to a second opto-electrical detector. The detectors are used to measure optical power and the emission wavelength, respectively, and yield two control signals for the injection current to control the laser. U.S. Pat. No. 6,122,301 to Tei et al. teaches the use of an interference filter and two detectors for measuring wavelength to produce a laser light source capable of stable emission. In U.S. Pat. No. 6,400,737 Broutin et al. teach an improvement to a wavelength-locker based on an interferometer in a closed-loop feedback control system to automatically adjust gain in a temperature tuned, wavelength stabilized laser. U.S. Pat. No. 6,289,028 to Munks et al. teaches a method and apparatus for monitoring and control of laser emission wavelength based on at least one optical filter. In accordance with the described method two separate beams are derived from the laser radiation by the one or more optical filters. A beam comparison element compares the first and second filtered beams and produces from them an error signal representative of the deviation of the wavelength of the laser from a set-point wavelength. The filters are set up so that the wavelength locking point is the wavelength at which the signals from the two detectors are equal. The filter tilt is chosen to set this wavelength (also called the “crossing wavelength”). Finally, in U.S. Pat. No. 4,172,663 Byer et al. teach the use of one or more interferometers in an optical wavelength meter.
Another prior art approach to wavelength locking is based on creating an interference pattern and determining changes in wavelength from the shifting of interference fringes detected by a number of detectors, e.g., a detector array. For example, U.S. Pat. No. 4,173,442 to Snyder teaches the use of a wedged interferometer (Fizeau-type interferometer) in a collimated beam to create a spatial interference pattern (i.e., interference fringes) in reflection mode. The fringes are imaged on a photoelectric receiver, typically a detector array, and the wavelength is determined from the measured location of the fringe minima (also called the “zero crossing”). The reader is referred to U.S. Pat. Nos. 3,967,211; 5,420,687 and 5,798,859 for still other approaches using the shift in an interference pattern to determine wavelength.
Unfortunately, the performance of prior art wavelength lockers is usually limited by difficulties encountered in providing high-precision temperature compensation over the operating range of the semiconductor laser without resorting to expensive on-board thermo-electric control. In wavelength lockers using etalons the temperature affects the optical path length in the etalon, which ultimately translates into wavelength errors for the locked laser. This design also suffers from the disadvantage that the high precision etalon (oftentimes a multi-element, air-spaced design) is costly, and that one cannot determine on which wavelength channel the laser is running, because wavelength lockers do not measure absolute wavelength.
The prior art also teaches the use of wavemeters. These devices determine the precise wavelength of the optical signal. In fact, in some cases wavemeters are capable of simultaneously measuring multiple wavelengths with high accuracy. Unfortunately, these devices are generally too large and costly for applications in telecommunications.
More recently, an approach for monitoring the precise wavelength of an optical beam has been proposed by Green in U.S. Pat. No. 6,331,892. Green uses an interferometer to create a constructive/destructive interference measured by a detector. The maximum and minimum length of the path traversed by the second beam in the interferometer are precisely controlled with a micro-positionable semiconductor retroreflector or mirror.
Using known oscillations of the retroreflector measured along the path of the second beam enables the user to obtain precise and repetitive measurements of the wavelength. Green also teaches that the detector measuring the interference pattern be implemented by three sensors for sampling the fringe pattern at three distinct locations when there is no micro-positionable retroreflector. These locations are preferably chosen such that the phase shift between signals from the individual detectors is approximately 90°, thus avoiding situations where the signal(s) from any detector(s) falls under a peak or valley of the interference pattern, since in these locations the sensor will be largely insensitive to wavelength variations, since the slope of the interference pattern at a fringe peak or valley is zero.
Although Green's teaching resolves a number of the prior art problems, it still requires a complex apparatus, including the oscillating retroreflector or three sensors in the interference pattern to obtain wavelength information.
Yet another teaching of a method and device for measuring and stabilization of a laser frequency using an interferometer and sensor signals derived from the interference pattern in 90° phase relationship is provided in U.S. Pat. No. 6,178,002 to Mueller-Wirts. The Muller-Wirts patent analyzes both the transmission and reflection signals of a wedge shaped interferometer to obtain correction of the laser frequency. However, this approach provides only for wavelength locking, since it is not capable of determining absolute wavelength.
In general, the last two prior art approaches provide high wavelength resolution, but require significant processing of the information from the detectors (for example curve fitting to determine spacing of fringes). The primary disadvantage of these methods and devices is that they cannot be used to easily and cheaply ascertain which particular channel the errors are associated with.